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Re: [Amps] Grid Dipping the Pi network in a new amp?

To: "Chuck Curran" <>
Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?
From: "Tom W8JI" <>
Date: Sun, 15 Jul 2007 20:59:07 -0400
List-post: <>
> I placed a 56 ohm resistor on the output, and a 1800 ohm 
> resistor on the
> tube side of the pi-network - nothing changed.  I re-read 
> the e-mails, and I
> had used 12" ( 30.5 cm ) leads for both resistors - not 
> good.  On Monday, I
> will do it with very short leads.  Tonight I have to grill 
> !

You can shorten the leads and it won't help. The biggest dip 
will come without any resistors.

You might try sticking the dip meter coil in towards the 
axis of the coils. Also make sure the vacuum caps are 
grounded to the chassis and all connections are solid. 
Sometimes a little film or paint or clear coating can 
insulate a ground, or you might have something anodized 
(makes a good insulator).

The only path that matters is the loop around through the 
tune and load caps and back through the coil. Imagine the 
tune cap in series with the load cap, and then the coil in 
series with them. That's the path that dips, and the reason 
you do not need (or want) resistors. Resistors lower Q, make 
the dip harder to see, and can mask an open loading 
capacitor path. A dip test means only the tank MIGHT work, 
not that it will.

If you want to do a resistor test, then you connect a 
resistor from anode to ground and look into the pi net 
output with an antenna analyzer. This is the ONLY test that 
tests the tank components for matching.

You could have a non-working tank that dips fine, especially 
with load resistance added.

You can't have a working tank that has a high SWR when you 
look backwards through it when it is tuned right.

You also can't have a working tank if it does not dip at all 
**without** resistors.

 In this case you don't get a dip at all, and that does 
indicate a tank problem.

73 Tom

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